Alien Swarm

Gameplay

Alien Swarm is a top-down shooter set at a 60-degree angle. Four players can join a single co-operative game, the aim of which is to progress through science fiction-themed levels while eliminating waves of aliens. Players can choose from 40 different pieces of equipment. The game includes persistent statistics, unlockable equipment, and achievements.

Classes

The game is class-based, with players choosing from the roles of Officer, Special Weapons, Medic, and Tech. Each class has two selectable characters, with differences in abilities.

Officer 

Grants a passive bonus to damage and damage resistance to nearby allies. He has access to the Vindicator, a class-restricted shotgun with an incendiary grenade launcher and has general all-around abilities. They are also capable of throwing extra amounts of explosive ordnance, and may find class-restricted ammo caches in the field for his Vindicator shotgun. The two playable characters are Sarge and Jaeger.

Special Weapons 

Brings raw damage to the table and starts out with a customized autogun with superior parameters, high magazine capacity, and auto-aiming ability. The Special Weapons gains access to a powerful minigun at a high enough level, and can pick up class-restricted ammo caches on the field. The two playable characters are Wildcat and Wolfe.

Medic 

Is the only class able to equip and deploy temporary healing beacons, and at a high enough level can use a medigun to selectively heal teammates and themselves. The two playable characters are Faith and Bastille.

Tech 

Carries a motion sensor, sets up sentry guns faster, welds doors faster, and is the only class capable of unlocking certain door panels or completing certain objectives by means of a hacking minigame. They also have access to a prototype assault rifle with auto-aiming capabilities and the ability to fire stun grenades. The two playable characters are Crash and Vegas.

Plot

In the game's only official campaign, Jacob's Rest, a swarm of invasive aliens have taken over a colonized planet in December 2052. Marines deployed by the Bloodhound dropship arrive to search for survivors and, if need be, to destroy the colony to prevent the aliens from spreading. The task force kills a large number of not only "normal" Swarm aliens, but alien eggs, large tumor-like growths, parasites and other creatures. They soon find out that the colonists have all been killed by the alien infestation. The marines then guide a thermonuclear bomb (originally meant for excavation purposes) through the complex and activate its timer. They return to the drop ship before the bomb detonates.

Development

Alien Swarm was announced in 2005, under the title of Alien Swarm: Infested, as a Source engine sequel to the original Unreal Tournament 2004 mod. However, by late 2007, the development blog had stopped updating, leaving its status uncertain. In July 2010 with the announcement of Alien Swarm, it was revealed that Valve had hired the team behind Alien Swarm, who had finished the mod between working on other Valve products such as Left 4 Dead and Portal 2.

Software development kit

An Alien Swarm software development kit (SDK) including buildable source code was released alongside the game. It is free to all users of Steam, rather than only to owners of existing Source games (as is the case with the 'mainline' Source SDK). This allows total conversion mods, which do not rely on content from other Valve games, to be free to all as well - a significant business decision that echoes the strategy of the Unreal Development Kit.

The SDK tools include a tile-based level designing tool called TileGen, which allows level designers to prefabricate rooms and hallways that can be either arranged in a certain way or randomly arranged based on generation rules. The tool does not replace the Valve Hammer Editor, the use of which is still required before a map can be distributed.

Reception

Alien Swarm received mixed to positive reviews upon release, with a 77/100 on aggregate review score website Metacritic.